Manual UMRead `make` recompile for PP file recognition

From a clean installation, at least under certain conditions, it appears e.g. as evidenced in point no. 1 of https://github.com/NCAS-CMS/cf-python/pull/87#issuecomment-645367474 that unless make is run for the UMRead library:

$ cd cf/umread_lib/c-lib
$ make

that PP files cannot be read in, e.g. as demonstrated in the test suite:

File "/home/runner/work/cf-python/cf-python/cf/read_write/read.py", line 928, in file_type
raise IOError("Can't determine format of file {}".format(filename))
    OSError: Can't determine format of file <filename>.pp

So we may need to add some steps to the setup.py to create the correct setup for PP processing.

Beginne mit setup.py und der UMRead-Bibliothek unter cf/umread_lib/c-lib; führe dort make mit einer sauberen Installation aus und reproduziere den PP-Dateifehler in der Testsuite. Vergleiche den Setup-Pfad mit dem manuellen Build und überprüfe anschließend, dass PP-Dateien erkannt werden, ohne make manuell auszuführen.